1. Field of the Invention
The present invention relates to a process and an apparatus capable of producing a polycrystalline semiconductor ingot of material such as silicon with less strain.
2. Description of the Related Art
A polycrystalline semiconductor of silicon or the like has received attention as a material for a solar cell or the like from the viewpoint of commercial production and resources. Most solar cells currently in use for solar power supply are silicon solar cells. The solar cell, however, still has the problem of high production cost. For utilizing this power source to a higher extent, it is necessary to further reduce the cost. Most recently, solar cells as power generators are based on single-crystal or amorphous silicon. So, further development of solar cells made of polycrystalline silicon is needed for cost reduction.
As a general process for producing a polycrystalline silicon semiconductor, conventionally solid silicon is charged in a crucible of silica (silicon dioxide SiO.sub.2) or the like, and after melting the solid silicon by heating, the melted semiconductor material is cast into a graphite crucible. In Japanese Examined Patent Publication JP-B2 57-21515, for example, there is disclosed a prior art of a semi-continuous cast furnace developed by Wacker-Chemitronie GmbH in Germany, wherein silicon is melted in a vacuum or in an inert gas in a silicon crucible, and is then poured into a mold made of graphite or the like by inclining the crucible. In Japanese Examined Patent Publication JP-B2 8-54115, there is disclosed a prior art of a heat exchange method developed by Crystal Systems, inc. in the U.S., wherein silicon is melted in vacuum in a silica crucible, and is then solidified directly--as it is. In Japanese Unexamined Patent Publication JP-A 62-260710, there is disclosed an improvement of the Wacker process, wherein a water-cooled steel sheet is used as the silicon melting crucible.
In the production of polycrystalline silicon ingot, most cases adopt a method of growing crystals in a crucible. In this method, however, a semiconductor polycrystal is required to be grown in a closed space. When an expansion in volume occurs during the solidification of crystals, which causes a stress at a contacting part between the solidified semiconductor polycrystal and an inner wall of the crucible, and a strain due to the stress is left unremoved in a thus produced ingot. The strain left in the ingot adversely affects the quality and reduces the optical or electrical characteristics of the semiconductor. This deteriorates the power generation efficiency in such applications as a solar cell. Also, an increased stress will cause mechanical cracking.